This invention relates to control circuitry for DC motors, and is more particularly directed to a control circuit for a capstan motor of a tape recording and/or reproducing device, especially a control circuit of a type supplying pulse-width modulated drive signals to the motor to drive the tape intermittently.
In video recorders, particularly video tape devices in which fields of video information are recorded on successive slant tracks on the tape, it is often useful to drive the tape intermittently so that different slant tracks can be repeatedly scanned, such as in slow-motion or still-frame reproduction. This is particularly useful, for example, when editing a video program recorded on tape.
On video tape, control signals are typically recorded in a longitudinal control track along an edge of the tape to indicate the position of respective associated slant tracks. That is, the control signals are situated in predetermined locations on the tape relative to the location of the slant tracks. Accordingly, the control signals, which are detected by a stationary control head as the tape is drawn past it, can be used to control stopping of the capstan during intermittent operation and to stop the tape at the location of a desired slant track. This ensures that the trace of a rotary head, carried on a rotating head wheel or drum for picking up the video signal, coincides accurately with the track. It is desirable that this be true for intermittent operation in both the forward and the reverse direction of transport of the tape.
A conventional capstan motor control circuit generates a drive pulse to effect intermittent motion of the capstan motor. This drive pulse commences (rises to high level) at the time a start signal is received and then ceases (drops to low level) at a later time estimated to correspond with the advancement of the tape by one slant track. A braking pulse occurs when the control signal for a desired track is detected, and is used to halt rotation of the capstan motor. This pulse has a width calculated be sufficient to stop the motor, but short enough not to cause reverse rotation thereof.
This conventional control circuit has not been capable of accurately halting the tape at the positions of the recorded slant video tracks. One reason for this is that the DC capstan motor has a torque ripple associated with it, typically 24 waves per rotation. Moreover, starting torque is typically much higher than output torque at normal capstan speed, so torque ripple at the time the appropriate control signal is detected (and the braking pulse is generated) pulse is further exaggerated, and results in unacceptable errors in the stopping position of the tape.
Because the torque ripple is typically 24 waves per rotation, it is desirable to control the motor with a signal that can vary at much smaller intervals, i.e., several tens of times the frequency of the torque ripple. Unfortunately, conventional intermittent drive control circuits have not been able to provide such a drive signal.